1. Field of the Invention
This invention relates to a liquid crystal alignment agent, more particularly to a vertical liquid crystal alignment agent having improved voltage holding ratio and reliability. The invention also relates to a liquid crystal alignment film formed from the liquid crystal alignment agent, and a liquid crystal display element including the liquid crystal alignment film.
2. Description of the Related Art
Nematic liquid crystal display elements are predominantly used in general liquid crystal display elements, and concrete examples of the nematic liquid crystal display elements actually used include: (1) a TN (Twisted Nematic) liquid crystal display element, in which a liquid crystal alignment direction of one side substrate is twisted at a 90 degree angle relative to a liquid crystal alignment direction of the other side substrate; (2) a STN (Super Twisted Nematic) liquid crystal display element, in which a liquid crystal alignment direction of one side substrate is twisted at an angle greater than 180 degrees relative to a liquid crystal alignment direction of the other side substrate; and (3) a TFT (Thin Film Transistor) liquid crystal display element which uses a thin film transistor.
The display panel made of the TN liquid crystal display element has a disadvantage of narrow viewing angle, and therefore, cannot satisfy the requirement for a wide viewing angle when applied to a large-sized display device, such as a liquid crystal display television.
To address the problem of the viewing angle, the following solutions are provided: (1) a TN liquid crystal display element used with an optical compensation film, (2) a VA (vertical alignment) liquid crystal display element used with an optical compensation film, (3) a MVA (Multi-domain Vertical Alignment) liquid crystal display element using vertical alignment and protrusion technologies, (4) an IPS (In-Plane Switching) liquid crystal display element, (5) an ECB (Electrically Controlled Birefringence) liquid crystal display element, and (6) an OCB (Optically Self-compensated Birefringence) liquid crystal display element. Among them, the MVA liquid crystal display element using vertical alignment and protrusion technologies and the IPS liquid crystal display element are relatively commonly used in the art.
In the MVA liquid crystal display element, the vertical alignment of the liquid crystal is primarily controlled by a liquid crystal alignment film so as to form the liquid crystal with a high pretilt angle. The liquid crystal alignment film is made from a liquid crystal alignment agent. Conventionally, the liquid crystal alignment agent is formulated by dissolving polyamic acid or polyimide in an organic solvent, and is then applied and cured on a substrate to form the liquid crystal alignment film. Two substrates each having the liquid crystal alignment film formed thereon are prepared and arranged to oppose each other with a space (cell gap). The peripheral portions of the two substrates are joined together with a sealing agent, liquid crystals are filled into the cell gap defined by the surfaces of the substrates and the sealing agent, and an injection hole is sealed up to form a liquid crystal cell.
JP 2002-162630 discloses a liquid crystal alignment film made of a polymer obtained by subjecting a diamine compound having an aromatic or cycloaliphatic side chain and a tetracarboxylic dianhydride compound to conduct a polymerization reaction.
It is increasingly required for a liquid crystal display device of large size to have enhanced reliability. However, such a requirement regarding reliability and voltage holding ratio can not achieved by conventional vertical liquid crystal alignment films. Therefore, it is still required in the art to develop a liquid crystal alignment agent which can be used to make a liquid crystal display element having superior reliability and high voltage holding ratio.